A typical hard disc drive or so-called Winchester disc drive is a unit for storing data consisting of a disc drive housing, a hard information storage disc, and a read/write head or slider for retrieving data from the hard disc. The slider is typically cantilevered over the hard disc from a supporting structure by a flexure arm.
In the typical operation of such a disc drive, the slider rests on a landing zone on the surface of the disc while the power is off. In operation, the drive unit is powered up and the disc begins to rotate. After the disc reaches a certain speed, the slider rises slightly off the landing zone under the influence of an air bearing generated by the rotation of the disc. However, until the slider rises, there is considerable friction associated with head drag on the disc which causes wear to both the slider and disc. Just breaking the head free from the surface of the disc on which the slider has been resting can require a considerable force to overcome the force which holds the two smooth surfaces together (called "stiction"). Additionally, when the unit is powered down, the same friction will occur between the surface of the head and the surface of the disc until the disc stops rotating.
In order to overcome this friction, discs are coated with a protective layer and lubricants are applied. Additionally, the discs typically require a dedicated landing zone where the slider can slide to a halt and rest when the unit is powered off. No data can be stored in the landing zone. Consequently, the amount of the data that can be stored on a disc is reduced. Moreover, a large disc drive motor is required to overcome the adverse frictional and stiction effects, and a motor brake is often necessary to stop the rotation of the disc when the motor is turned off to reduce frictional wear and motor size requirements.
Thus, it can be appreciated that it would be desirable to raise the slider during power-up and keep it raised for a brief period even after power-down until the disc has essentially come to a halt to eliminate the adverse effects of friction.
In a number of prior-art patents including U.S. Pat. No. 4,684,913, it has been proposed to take advantage of a shape memory metal phenomenon in raising and lowering the slider or actuator. The phenomenon of shape memory is, of course, already well understood. It is based on the thermoelastic martensitic transformation which will be explained hereunder. A shape memory alloy, such as Ti--Ni alloy, has a high temperature austenitic phase wherein the crystal structure is body center cubic. When cooled below its transformation temperature, the austenitic structure undergoes a diffusionless shear transformation into a highly twinned martensitic crystal structure. In the martensite phase, the alloy is easily deformed by the application of a small external force. When the alloy is heated through its transformation temperature, the martensitic phase is elastically returned to the former austenitic phase (inverse transformation) according to a given ordered crystal and orientation law. The alloy has the property of offering an exceedingly large recovery force when returning to this austenitic phase. Therefore, the employment of a resilient force as a bias force for deformation of the martensitic phase alloy at a low temperature permits the alloy to be used as a reversible actuator with temperature cycling. Further, since the recovery force which is generated with the return to the austenitic phase is quite large, it is possible to take advantage of the recovery force to do work.
It is important to the design of slider lifters to provide a device which raises and lowers the slider without requiring modification of the design of the flexure arm. Avoiding modification of the existing design of the flexure arm is a primary aspect of a successful device for loading and unloading the head. The lack of commercial success of prior-art designs is due in significant part to their failures in this area. The reason is that in a conventional floating head slider, the head is expected to float in a stable fashion about 0.2 micrometers over the surface of the disc which is rotating at constant speed. Therefore, the combined flexure arm and slider are very sensitive as to their loading and air foil characteristics. A great deal of time and effort have gone into the flexure and slider designs. Any design that requires modification of the flexure arm is looked upon with great disfavor.
Two prior art patents representative of the use of shape memory alloy technology in load/unload suspension devices for magnetic disc apparatus are U.S. Pat. No. 4,605,979, Inoue, and U.S. Pat. No. 4,684,913, Yaeger. Both of these, as can be immediately seen from an inspection of the figures, incorporate the use of shape memory alloys to attempt to provide a more efficient system for loading and unloading a transducer head from a disc surface. However, they each utilize a relatively high profile device which requires a significant spacing between adjacent discs. In present disc drive technology, such spacing is simply no longer available. To maximize the capacity of disc drives being manufactured, discs are spaced as closely as the supporting flexure will allow.
Therefore, the problem presented in the design of the present invention is to incorporate the strong recovery force affording by shape memory metals in a head unloading design without requiring significant modification of the existing design characteristics of the flexure arm. Another requirement is to incorporate a flexure utilizing shape memory alloys to load and unload the head while maintaining an extremely low flexure profile to minimize spacing between the vertically-spaced discs.